Lithographic apparatus and device manufacturing method

ABSTRACT

A map of the surface of a substrate is generated at a measurement station. The substrate is then moved to where a space between a projection lens and the substrate is filled with a liquid. The substrate is then aligned using, for example, a transmission image sensor and, using the previous mapping, the substrate can be accurately exposed. Thus the mapping does not take place in a liquid environment.

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/853,030, filed Aug. 9, 2010, which is a divisional ofco-pending U.S. patent application Ser. No. 12/318,036, filed Dec. 19,2008, now U.S. Pat. No. 7,795,603, issued on Sep. 14, 2010, which is acontinuation of co-pending U.S. patent application Ser. No. 11/371,235,filed Mar. 9, 2006, now U.S. Pat. No. 7,482,611, issued on Jan. 27,2009, which is a continuation of co-pending U.S. application Ser. No.10/705,816, filed Nov. 12, 2003, now U.S. Pat. No. 7,193,232, issued onMar. 20, 2007, which claims priority from European patent applicationsEP 02257822.3, filed Nov. 12, 2002, and EP 03253692.2, filed Jun. 11,2003, all the foregoing applications herein incorporated in theirentirety by reference.

FIELD

The present invention relates to immersion lithography.

BACKGROUND

The term “patterning device” as here employed should be broadlyinterpreted as referring to means that can be used to endow an incomingradiation beam with a patterned cross-section, corresponding to apattern that is to be created in a target portion of the substrate; theterm “light valve” can also be used in this context. Generally, the saidpattern will correspond to a particular functional layer in a devicebeing created in the target portion, such as an integrated circuit orother device (see below). Examples of such a patterning device include:

-   -   A mask. The concept of a mask is well known in lithography, and        it includes mask types such as binary, alternating phase-shift,        and attenuated phase-shift, as well as various hybrid mask        types. Placement of such a mask in the radiation beam causes        selective transmission (in the case of a transmissive mask) or        reflection (in the case of a reflective mask) of the radiation        impinging on the mask, according to the pattern on the mask. In        the case of a mask, the support structure will generally be a        mask table, which ensures that the mask can be held at a desired        position in the incoming radiation beam, and that it can be        moved relative to the beam if so desired.    -   A programmable mirror array. One example of such a device is a        matrix-addressable surface having a viscoelastic control layer        and a reflective surface. The basic principle behind such an        apparatus is that (for example) addressed areas of the        reflective surface reflect incident light as diffracted light,        whereas unaddressed areas reflect incident light as undiffracted        light. Using an appropriate filter, the said undiffracted light        can be filtered out of the reflected beam, leaving only the        diffracted light behind; in this manner, the beam becomes        patterned according to the addressing pattern of the        matrix-addressable surface. An alternative embodiment of a        programmable mirror array employs a matrix arrangement of tiny        mirrors, each of which can be individually tilted about an axis        by applying a suitable localized electric field, or by employing        piezoelectric actuation means. Once again, the mirrors are        matrix-addressable, such that addressed mirrors will reflect an        incoming radiation beam in a different direction to unaddressed        mirrors; in this manner, the reflected beam is patterned        according to the addressing pattern of the matrix-addressable        mirrors. The required matrix addressing can be performed using        suitable electronic means. In both of the situations described        hereabove, the patterning device can comprise one or more        programmable mirror arrays. More information on mirror arrays as        here referred to can be gleaned, for example, from United States        patents U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patent        applications WO 98/38597 and WO 98/33096, which are incorporated        herein by reference. In the case of a programmable mirror array,        the said support structure may be embodied as a frame or table,        for example, which may be fixed or movable as required.    -   A programmable LCD array. An example of such a construction is        given in U.S. Pat. No. 5,229,872, which is incorporated herein        by reference. As above, the support structure in this case may        be embodied as a frame or table, for example, which may be fixed        or movable as required.

For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask andmask table; however, the general principles discussed in such instancesshould be seen in the broader context of the patterning device ashereabove set forth.

Lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, the patterningdevice may generate a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto a target portion(e.g. comprising one or more dies) on a substrate (silicon wafer. LCD,mask etc) that has been coated with a layer of radiation-sensitivematerial (resist). In general, a single wafer will contain a wholenetwork of adjacent target portions that are successively irradiated viathe projection system, one at a time. In current apparatus, employingpatterning by a mask on a mask table, a distinction can be made betweentwo different types of machine. In one type of lithographic projectionapparatus, each target portion is irradiated by exposing the entire maskpattern onto the target portion at one time; such an apparatus iscommonly referred to as a wafer stepper. In an alternative apparatus-commonly referred to as a step-and-scan apparatus—each target portionis irradiated by progressively scanning the mask pattern under theprojection beam in a given reference direction (the “scanning”direction) while synchronously scanning the substrate table parallel oranti-parallel to this direction; since, in general, the projectionsystem will have a magnification factor M (generally <1), the speed V atwhich the substrate table is scanned will be a factor M times that atwhich the mask table is scanned. More information with regard tolithographic devices as here described can be gleaned, for example, fromU.S. Pat. No. 6,046,792, incorporated herein by reference.

In a manufacturing process using a lithographic projection apparatus, apattern (e.g. in a mask) is imaged onto a substrate that is at leastpartially covered by a layer of radiation-sensitive material (resist).Prior to this imaging step, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g. anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4, incorporated herein by reference.

For the sake of simplicity, the projection system may hereinafter bereferred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens”.Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Dual stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO98/40791, incorporated herein by reference in their entirety.

The lithographic industry is constantly trying to reduce feature sizeson silicon substrates in order to manufacture ever more complexintegrated circuits. The feature sizes are limited by the effect ofdiffraction and thus the resolution of a particular system of numeralaperture NA using a wavelength λ is:

$W = {k\frac{\lambda}{N\; A}}$

where k is a pre-factor. The numerical aperture NA is n sin θ where n isthe refractive index of the transmissive substance.

Hence to decrease the resolution, the wavelength can either be reducedor the numerical aperture increased. It has been proposed to immerse thesubstrate in a liquid having a relatively high refractive index, e.g.water, so as to fill a space between the final element of the projectionsystem and the substrate. The point of this is to enable imaging ofsmaller features since the exposure radiation will have a shorterwavelength in the liquid. (The effect of the liquid may also be regardedas increasing the effective NA of the system).

However, submersing the substrate or substrate and substrate table in abath of liquid (see for example U.S. Pat. No. 4,509,852, herebyincorporated in its entirety by reference) may mean that there is alarge body of liquid that must be accelerated during a scanningexposure. This may require additional or more powerful motors andturbulence in the liquid may lead to undesirable and unpredictableeffects.

One of the solutions proposed is for a liquid supply system to provideliquid in a localized area between the final element of the projectionsystem and the substrate (the substrate generally has a larger surfacearea than the final element of the projection systems). One way whichhas been proposed to arrange for this is disclosed in PCT patentapplication WO 99/49504, hereby incorporated in its entirety byreference. As illustrated in FIGS. 4 and 5, liquid is supplied by atleast one inlet IN onto the substrate, preferably along the direction ofmovement of the substrate, relative to the final element, and is removedby at least one outlet OUT after having passed under the projectionsystem. That is, as the substrate is scanned beneath the element in a −Xdirection, liquid is supplied at the +X side of the element and taken upat the −X side. FIG. 4 shows the arrangement schematically in whichliquid is supplied via inlet IN and is taken up on the other side of theelement by outlet OUT which is connected to a low pressure source. Inthe illustration of FIG. 4 the liquid is supplied along the direction ofmovement of the substrate relative to the final element, though thisdoes not need to be the case. Various orientations and numbers of in andout-lets positioned around the final element are possible, one exampleis illustrated in FIG. 5 in which four sets of an inlet with an outleton either side are provided in a regular pattern around the finalelement to form a liquid reservoir.

SUMMARY

Immersion lithography is an embryonic technology and there remain manyproblems in its practical application. This patent application isconcerned in particular with alignment and leveling of a substrate.Conventionally alignment and leveling is performed with the substrate inthe field of view of the projection system (i.e. at an exposurestation). However there is not a lot of space for alignment or levelmeasurement apparatus in and around an immersion liquid reservoir so theadaptation is likely to be complex or the accuracy can be compromised.Furthermore, the presence of liquid near the alignment and levelmeasurement apparatus can degrade the performance of the apparatus.

Accordingly, it may be advantageous to provide, for example, a methodand apparatus for accurately aligning and/or leveling a substrate in animmersion lithography apparatus.

According to an aspect, there is provided a lithographic projectionapparatus comprising:

a support structure configured to hold a patterning device, thepatterning device configured to pattern a beam of radiation according toa desired pattern;

a substrate table configured to hold a substrate;

a projection system configured to project the patterned beam onto atarget portion of the substrate;

a liquid supply system configured to provide a liquid, through whichsaid beam is to be projected, in a space between said projection systemand said substrate; and

a measurement system configured to measure, not through said liquid,locations of points on said substrate.

The position of points on the substrate are thus measured without thepresence of liquid and, in an embodiment, outside the immersion system.Alternatively, the measurements could take place while a target portionof the substrate is submerged in liquid, i.e. the measurements takeplace through liquid, but not the same liquid as supplied by the liquidsupply system to fill a space between the projection system and thesubstrate. The position of points on the substrate would therefore bemeasured with liquid between the measurement system and the substrate,the liquid would then be removed before moving the substrate (andsubstrate table) to the focal point of the projection system where theliquid supply system would supply liquid to fill a space between theprojection system and the substrate prior to exposure taking place. Asecond liquid supply system may be present in the vicinity of themeasurement system.

A possible advantage is that there is better flow in the liquidreservoir because the measurement system is no longer in or around thereservoir crowding the projection system and the performance of themeasurement system is not degraded by the presence of liquid.Furthermore smooth flow conditions in the liquid reservoir are desiredas there is no change in the apparatus leading to rough edges. Usingthis method, measurement systems not specifically adapted for immersionlithography can be used without complex adaptation. A further advantageof this measurement system is that any improvements to such measurementsystems used outside of the immersion lithography field can easily andautomatically be incorporated into the immersion system.

The measurement system, in an embodiment, comprises an alignment systemconfigured to measure the location (in the x, y and R_(z) directions) ofa plurality of alignment marks on said substrate. According to anembodiment, said substrate table has a reference and said measurementsystem measures the location of said reference not through said liquidof said supply system. The location of the alignment marks may in anembodiment be measured relative to said reference on said substratetable to enable a map of alignment marks relative to the reference to bebuilt up.

According to an embodiment, the measurement system comprises a levelsensor configured to measure the height and/or tilt (i.e. measuring inthe z, R_(x) and R_(y) directions) of points on said substrate. Thus,level measurement of the substrate, which is conventionally undertaken“on-the-fly” at the exposure station, can be achieved outside the liquidreservoir.

In an embodiment, the lithographic projection apparatus can have anexposure station at which said substrate may be exposed and a separatemeasurement station, said measurement system being provided at saidmeasurement station and said substrate table being movable between saidexposure and measurement stations. Furthermore, there can be a pluralityof substrate tables, each movable between an exposure station and ameasurement station. While one substrate table is being mapped, a secondsubstrate table can be exposed. Substrate throughput may therefore beincreased, making the apparatus more efficient and improving the cost ofownership.

According to an embodiment, said reference is a transmission imagesensor.

In an embodiment, the alignment system measures displacement in twolinear perpendicular directions and rotation within the plane defined bythe two perpendicular directions.

In an embodiment, said liquid supply system is configured to providesaid liquid to a space between a final lens of said projection systemand said substrate.

According to a further aspect, there is provided a device manufacturingmethod comprising:

providing a liquid in a space between a projection system and asubstrate;

measuring the locations of points on a substrate using a measurementbeam projected from a measurement system but not projected through saidliquid; and

projecting a patterned beam of radiation, through said liquid, onto atarget portion of the substrate using the projection system.

Although specific reference may be made in this text to the use of theapparatus described herein in the manufacture of ICs, it should beexplicitly understood that such an apparatus has many other possibleapplications. For example, it may be employed in the manufacture ofintegrated optical systems, guidance and detection patterns for magneticdomain memories, liquid-crystal display panels, thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “reticle”, “wafer”or “die” in this text should be considered as being replaced by the moregeneral terms “mask”, “substrate” and “target portion”, respectively.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm).

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in which:

FIG. 1 depicts a lithographic projection apparatus according to anembodiment of the invention;

FIG. 2 depicts a detail of a lithographic projection apparatus accordingto an embodiment of the invention;

FIG. 3 depicts the same details of the lithographic projection apparatusas FIG. 2 at a different stage in the exposure process according to anembodiment of the invention;

FIG. 4 depicts an alternative liquid supply system according to anembodiment of the invention; and

FIG. 5 is an alternative view of the liquid supply system of FIG. 4according to an embodiment of the invention.

In the Figures, corresponding reference symbols indicate correspondingparts.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic projection apparatusaccording to a particular embodiment of the invention. The apparatuscomprises:

-   -   a radiation system Ex, IL, for supplying a projection beam PB of        radiation (e.g. UV radiation), which in this particular case        also comprises a radiation source LA;    -   a first object table (mask table) MT provided with a mask holder        for holding a mask MA (e.g. a reticle), and connected to first        positioning means for accurately positioning the mask with        respect to item PL;    -   a second object table (substrate table) WT provided with a        substrate holder for holding a substrate W (e.g. a resist-coated        silicon wafer), and connected to second positioning means for        accurately positioning the substrate with respect to item PL;    -   a projection system (“lens”) PL (e.g. a refractive lens system)        for imaging an irradiated portion of the mask MA onto a target        portion C (e.g. comprising one or more dies) of the substrate W.

As here depicted, the apparatus is of a transmissive type (e.g. has atransmissive mask). However, in general, it may also be of a reflectivetype, for example (e.g. with a reflective mask). Alternatively, theapparatus may employ another kind of patterning device, such as aprogrammable mirror array of a type as referred to above.

The source LA (e.g. a laser-produced or discharge plasma source)produces a beam of radiation. This beam is fed into an illuminationsystem (illuminator) IL, either directly or after having traversedconditioning means, such as a beam expander Ex, for example. Theilluminator IL may comprise adjusting means AM for setting the outerand/or inner radial extent (commonly referred to as σ-outer and σ-inner,respectively) of the intensity distribution in the beam. In addition, itwill generally comprise various other components, such as an integratorIN and a condenser CO. In this way, the beam PB impinging on the mask MAhas a desired uniformity and intensity distribution in itscross-section.

It should be noted with regard to FIG. 1 that the source LA may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source LA is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, theradiation beam which it produces being led into the apparatus (e.g. withthe aid of suitable directing mirrors); this latter scenario is oftenthe case when the source LA is an excimer laser. The current inventionand claims encompass both of these scenarios.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed the mask MA, the beam PB passes through theprojection system PL, which focuses the beam PB onto a target portion Cof the substrate W. With the aid of the second positioning means (andinterferometric measuring means IF), the substrate table WT can be movedaccurately, e.g. so as to position different target portions C in thepath of the beam PB. Similarly, the first positioning means can be usedto accurately position the mask MA with respect to the path of the beamPB, e.g. after mechanical retrieval of the mask MA from a mask library,or during a scan. In general, movement of the object tables MT, WT willbe realized with the aid of a long-stroke module (course positioning)and a short-stroke module (fine positioning), which are not explicitlydepicted in FIG. 1. However, in the case of a wafer stepper (as opposedto a step-and-scan apparatus) the mask table MT may just be connected toa short stroke actuator, or may be fixed in the XY plane.

The depicted apparatus can be used in two different modes:

1. In step mode, the mask table MT is kept essentially stationary, andan entire mask image is projected at one time (i.e. a single “flash”)onto a target portion C. The substrate table WT is then shifted in the xand/or y directions so that a different target portion C can beirradiated by the beam PB;

2. In scan mode, essentially the same scenario applies, except that agiven target portion C is not exposed in a single “flash”. Instead, themask table MT is movable in a given direction (the so-called “scandirection”, e.g. the y direction) with a speed v, so that the projectionbeam PB is caused to scan over a mask image; concurrently, the substratetable WT is simultaneously moved in the same or opposite direction at aspeed V=Mv, in which M is the magnification of the projection system PL(typically, M=1/4 or 1/5). In this manner, a relatively large targetportion C can be exposed, without having to compromise on resolution.

In FIG. 2 the substrate table WT is at a measurement station wherealignment and/or level measurement take place. The substrate table isprovided with a reference F1, sometimes referred to as a fiducial, whichcan comprise a plate etched through with a pattern corresponding to astandard alignment mark underneath which is a radiation sensor, alsoknown as a transmission image sensor, responsive to radiation. At themeasurement station, the substrate table WT is moved to detect thereference F1 using an alignment system within the measurement system 30and then to detect the alignment marks on the substrate W therebyenabling the location (in directions x, y and R_(z)) of the substratealignment marks to be found. In an embodiment, the location of thealignment marks are measured and determined relative to the reference.

Level measurement of the substrate then occurs at the measurementstation. In order to measure the level of the substrate, a leveling beam(projected from the measurement system 30) can be used that traverses afirst grating prior to reflection by the substrate W. A second gratingis then placed in the path of the leveling beam after reflection by thesubstrate W. The extent to which the images of the first and secondgratings coincide is measured by a level measurement sensor and isdetermined by the height and/or tilts of the substrate W (the z, R_(x)and R_(y) coordinates are thus determined). For a further description oflevel measurement of substrates reference is made to European patentapplication EP 0502583. Hence, using data from the alignment of thesubstrate and the level measurement of the substrate, a map of thesubstrate can be generated.

As shown in FIG. 3, substrate table WT is then moved to the separateexposure station where a liquid supply 18 is provided to supply liquid(e.g. water) to a space between the projection system PL and thesubstrate table WT to form a liquid reservoir 10. In this example, thereservoir 10 forms a contactless seal to the substrate around the imagefield of the projection system PL so that liquid is confined to fill aspace between the substrate surface and the final element of theprojection system PL. A seal member 12, positioned below and surroundingthe final element of the projection system PL, borders the reservoir 10and comprises the liquid supply 18. The seal member 12 extends a littleabove the final element of the projection system and has an innerperiphery that at the upper end closely conforms to the step of theprojection system or the final element thereof and may, e.g., be round.At the bottom, the inner periphery closely conforms to the shape of theimage field, e.g., rectangular though this need not be the case. Liquidis brought into the space below the projection system and within theseal member 12 and the liquid level rises above the final element of theprojection system PL so that a buffer of liquid is provided.

A gas seal 16, formed between the bottom of the seal member 12 and thesurface of the substrate W, confines the liquid in the reservoir. Thegas seal is formed by gas, e.g. air or synthetic air but in anembodiment, N₂ or another inert gas, provided under pressure via inlet15 to the gap between seal member 12 and the substrate W and extractedvia first outlet 14. An overpressure on the gas inlet 15, vacuum levelon the first outlet 14 and geometry of the gap are arranged so thatthere is a high-velocity gas flow inwards that confines the liquid.

In an embodiment, the liquid reservoir defined by inlet(s) IN andoutlet(s) OUT as shown in FIGS. 4 and 5 can be similarly applied. Insuch a case, a measurement station can be provided as well as anexposure station comprising inlet(s) IN and outlet(s) OUT.

To ascertain the exact position of the substrate table WT at theexposure station the reference F1 is scanned in three dimensions throughthe aerial image of an alignment mark on the mask MA. The maximum signalis returned when the reference is aligned with the image of the mark onthe mask in the plane of best focus. Using the map of the substrate Wgenerated at the measurement station, the location, height and/or tiltof positions on the substrate W are therefore known. In order to trackthe movements of the substrate table WT, suitable position measurementsdevices can be used such as an interferometer beam projected towards oneor more sides of the substrate table WT. A particular point on thesubstrate table can be placed at the focal point of the projectionsystem PL and exposure of a target portion C of the substrate W can takeplace.

Once exposure of the substrate W is completed it is then removed forfurther processing and a new substrate placed on substrate table WT. Thesubstrate table with the new substrate returns to the measurementstation and the process can be repeated.

Prior to the substrate table WT leaving the exposure station, the liquidreservoir can be emptied, for example in the case shown in FIGS. 2 and3, by reducing the gas inlet pressure and allowing the liquid to besucked out by the vacuum system or, for example in the case shown inFIGS. 4 and 5, by discontinuing flow of liquid onto the substratethrough inlet IN and allowing the liquid to be sucked out by outlet OUT.

To ascertain the exact position of the substrate table WT, the positionof the transmission image sensor described above can be sensed throughthe liquid, or alternatively not through the liquid and a correctionapplied.

According to an embodiment, there are at least two substrate tables,each bearing a reference, and while one substrate table is at themeasurement station the other is at the exposure station. The substratetables are movable between an exposure station and a measurementstation.

Instead of using the reference mark F1 and the projection system toalign the substrate, off-axis measurement can be used. The referencemark F1 can be aligned using another system near the projection systemPL. Alternatively, a different reference and a different system, forexample one with an axis perpendicular to the projection axis of theprojection system can be used. Further description of such off-axismeasurement can be found in European patent application publication EP0906590.

Alternatively, if the substrate table is above the projection system(i.e. the projection system is upside down compared to FIG. 1) theliquid in liquid reservoir 10 may not need to be completely removed andcould just be refilled as necessary.

In an alternative detection embodiment, there is no separate measurementstation. Detection and measurement of an alignment mark takes place atthe exposure station but with no liquid in reservoir 10. The liquidreservoir 10 is then filled up and exposure takes place. Similarly levelmeasurement can take place at the exposure station with no liquid inreservoir 10. These measurements can be either off-axis or on-axis.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

The invention claimed is:
 1. A lithographic apparatus comprising: anexposure station including a projection system configured to project apatterned beam of radiation through liquid onto a target portion of asubstrate held on a first substrate table; a measurement stationconfigured to measure a substrate held on a second substrate table,wherein measurement of the substrate at the measurement station is notperformed through said liquid adjacent the substrate; and a positioningsystem configured to move the second substrate table from themeasurement station to the exposure station so as to position thesubstrate held on the second substrate table into an optical path of theprojection system.
 2. The lithographic apparatus of claim 1, wherein thepositioning system is configured to move the first substrate table fromthe exposure station to the measurement station.
 3. The lithographicapparatus of claim 1, wherein measurement of the substrate at themeasurement station is performed through a second liquid adjacent thesubstrate, the second liquid different from said liquid.
 4. Thelithographic apparatus of claim 1, wherein said liquid is in contactwith a last element of the projection system and the substrate duringprojection of the patterned beam of radiation.
 5. The lithographicapparatus of claim 1, wherein the measurement station is configured tomeasure a height, a tilt, or both, of each of a plurality of points onthe substrate.
 6. The lithographic apparatus of claim 1, wherein themeasurement station is configured to measure the substrate held on thesecond substrate table in the absence of liquid adjacent the substrate.7. The lithographic apparatus of claim 1, wherein the second substratetable comprises a reference and the measurement station comprises analignment system configured to measure a location of the reference. 8.The lithographic apparatus of claim 7, comprising an alignment systemconfigured to measure a location of the reference at the exposurestation, wherein if the reference is not measured through liquid acorrection is applied.
 9. The lithographic apparatus of claim 1, whereinthe exposure station comprises a liquid supply system configured toprovide liquid onto the target portion of the substrate.
 10. Thelithographic apparatus of claim 1, wherein measurement of the substrateat the measurement station is performed at least partially overlappingin time with exposure of the substrate at the exposure station.